1. Field of the Invention
The present invention relates to a substrate for an inkjet head for printing a print medium by ejecting ink and an inkjet head.
2. Description of the Related Art
At present, there is widely used an inkjet printing apparatus wherein ink in liquid chambers is heated by energizing heating resistors, whereby film boiling in the heated ink causes foaming in the ink, and its foaming energy causes ink droplets to be ejected from ejection ports. During the printing by such an inkjet printing apparatus, some regions of the heating resistors are occasionally affected by physical action such as the impact of cavitation caused by ink foaming, shrinkage, and defoaming in the regions of the heating resistors. Moreover, since the heating resistors are kept at a high temperature during the ejection of ink, some regions of the heating resistors are occasionally affected by chemical action such as adhesion and deposition of ink components to and on the surfaces of the heating resistors. To protect the heating resistors from the physical action or the chemical action, a protection layer is occasionally disposed over the heating resistors to cover the heating resistors.
Generally, the protection layer is disposed at a position where the protection layer contacts ink. Accordingly, in a case where electricity flows through the protection layer, an electrochemical reaction occasionally occurs between the protection layer and ink, thereby damaging the function of the protection layer. In order to prevent this, an insulating layer is disposed between the heating resistor and the protection layer so that part of the electricity supplied to the heating resistor does not flow through the protection layer.
However, there is a case where the function of the insulating layer is damaged for some reason and electricity directly flows from the heating resistor or wiring to the protection layer, thereby causing a short circuit. In a case where part of the electricity supplied to the heating resistor flows through the protection layer, an electrochemical reaction occasionally occurs between the protection layer and ink, thereby degenerating the protection layer. In a case where the protection layer is disposed across the plurality of heating resistors, the entire protection layer may be affected.
Accordingly, it is considered that individual sections of the protection layer provided to correspond to the plurality of heating resistors and a common section of the protection layer commonly connecting the individual sections are connected by fuse sections provided on part of the protection layer. With the fuse sections provided on part of the protection layer, in a case where a current flows through the protection layer, an electrical connection can be cut so that the current is prevented from flowing through the other part of the protection layer.
Japanese Patent No. 3828728 discloses an example of an inkjet head in which a fuse section forms a part of the inkjet head. Japanese Patent No. 3828728 discloses a fuse for dissipating charge on a protection layer to other portions and cutting the electrical connection between the protection layer and a positive voltage pad at predetermined timing in order to reduce the impact of electrostatic discharge (ESD) on a print system in case that the electrostatic discharge is occurred.
However, Japanese Patent No. 3828728 discloses using a field effect transistor (FET) as the fuse provided between the protection layer and the positive voltage pad in the inkjet head. Japanese Patent No. 3828728 further discloses that the fuse is destroyed at predetermined timing, thereby cutting the electrical connection between the protection layer and the positive voltage pad. In this case, a relatively large energy is required for cutting the electrical connection between the protection layer and the positive voltage pad. Accordingly, part of the current flows from the protection layer to other portions before the fuse cuts the electrical connection, which occasionally affects other regions around the protection layer.